Last year Congress gave NASA until mid-January 2011 to pick a design for the rocket, known as the Space Launch System, that will carry astronauts on deep space missions. NASA still has not made an official decision, and members of the House of Representatives' Committee on Science, Space and Technology took Bolden to task.

"We've waited for answers that have not come. We've pleaded for answers that have not come," committee chairman Ralph Hall (R-Texas) told Bolden. "We've run out of patience."

Hall said the committee reserves the right to open an investigation into the delays, but he hopes it won't come to that.

"The White House has done you wrong, but nonetheless you have to answer for these continued failures," said Hall, whose panel is made up of 23 Republicans and 17 Democrats.

Though many of the Republicans blamed President Barack Obama for NASA's perceived problems, Bolden said the buck stops with him.

"You have the right guy here to criticize," Bolden said. "I am the leader of America's space program."

Bolden also said NASA has a design in mind and is simply awaiting estimates of what it will cost to build.

NASA's new deep space plan

Last year Obama laid out a deep space exploration plan for the National Aeronautics and Space Administration: Get an astronaut to an asteroid by 2025, then to Mars by the mid-2030s.

The Space Launch System is a key part of this plan, as is a crewed deep space vehicle. The basic architecture for both of these components was supposed to be decided by January.

In May, NASA announced that the spaceship, which it calls the Multi-Purpose Crew Vehicle, would be based heavily on the Orion spacecraft. NASA originally developed Orion for President George W. Bush's moon-oriented Constellation program; Obama canceled the program last year.

But the space agency still hasn't officially announced the architecture for the new rocket that will launch the crew vehicle to deep space.

NASA maintained at various times that a decision was coming in the spring, then in June, then in early July. NASA associate administrator Lori Garver said last week the agency hopes to make an announcement by late summer.

The repeated delays have not gone over well with many members of Congress, who want NASA to show it has solid plans for continuing the United States' global leadership in human spaceflight following the retirement of the space shuttle program. The iconic program will wind down after 30 years of operation when the shuttle Atlantis touches down July 21. [Photos: NASA's Last Shuttle Mission in Pictures]

If NASA doesn't demonstrate a clear path forward, some Congress members said, U.S. leadership is threatened. Many people in the aerospace industry, for example, might move on, fearing that jobs will dry up for the long haul.

"I firmly believe that if we lose this talent, it won't be just to another state or another agency," said Rep. Eddie Bernice Johnson (D-Texas). "It'll be to another country."

Critical decision looms

In testimony that was at times emotional — he choked up on two separate occasions when discussing the shuttle program and the future of American spaceflight— Bolden acknowledged the frustration but said the extra time NASA is taking is warranted.

"I share that interest and urgency," Bolden said. "But we cannot rush a critical decision that will drive NASA's activities for decades."

Bolden said NASA chose a reference design for the heavy-lift vehicle last month and has submitted it to two separate organizations — the federal Office of Management and Budget and the consulting firm Booz Allen Hamilton — for cost assessments. These cost assessments aren't complete yet, and that's what's causing the most recent delay.

Bolden said he wouldn't be comfortable making a formal announcement without having those cost assessments. He reiterated Garver's estimated timeframe of late summer, but said he would take even longer if need be.

"It would be irresponsible to proceed further until we at least have good estimates," Bolden said. "This will most likely be the most important decision I make as the NASA administrator, and I want to get it right."

The future is bright

Bolden stressed that NASA is still on track to achieve the exploration goals the president laid out for it last year. He said the rocket and crew vehicle could start making unmanned test flights by 2017, with manned operations perhaps launching around 2020 or so.

And he reminded the committee about NASA's efforts to encourage commercial companies to develop their own spaceships, which could soon be transporting cargo and crew to the International Space Station.

At least one of these private craft should be operating in low-Earth orbit by 2015, Bolden said. Therefore, America's dependence on Russian Soyuz vehicles to fill this space-taxi role — a state of affairs lamented by several Congress members today — should last just four years or so.

For these reasons and more, Bolden said, the future of U.S. manned spaceflight is bright.

"We are not abandoning human spaceflight," he said. "American leadership in space will continue for at least the next half century because we have laid the foundation for success."

You can follow SPACE.com senior writer Mike Wall on Twitter: @michaeldwall. Follow SPACE.com for the latest in space science and exploration news on Twitter @Spacedotcom and on Facebook.